Automatic transmissions for motor vehicles usually have a mechanically operating parking brake device where, in parking position for example, a pawl detent engages in a toothing of a parking brake gear which is connected with the output of the transmission and thus with an axle of the vehicle. Mechanical and electrohydraulic systems are known as operative connections between parking brake and the control device thereof in the interior of the motor vehicle. Mechanical operative connections are constructed, for example, as Bowden cable, between the selector lever fixed to the body and the electrohydraulic transmission control disposed on the automatic transmission and the parking brake actuating device located in the transmission housing. A purely electric operative connection between the control device of the automatic transmission in the interior of the vehicle and the electrohydraulic transmission control implies a reaction of the electric signals to the parking brake actuating device in a mechanical movement of the detent pawl.
As an essential advantage of the purely electrically designed operative connection between the control device of the automatic transmission and the electrohydraulic transmission control, the site of the control device in the interior of the vehicle can be freely chosen. Another essential advantage is the acoustic uncoupling of the control device from the drive train of the motor vehicle.
DE 43 22 523 A1, for example, describes an electrohydraulic control device for an automatic transmission of a motor vehicle having one electrically constructed operative connection between the control device of the automatic transmission in the interior of the vehicle and the electrohydraulic transmission control in which the operating steps D, R and N are activated by pressurization of individual shifting members while the operating step P is activated by pressureless shift of all shifting members. This arrangement results in that the parking brake is activated upon drop of the pressure not only in all operating step P but in all operating steps of the selector lever. The parking brake is activated here via a spring accumulator; the deactivation of the parking brake is hydraulic. The pressure required for deactivation is made available via the oil pressure circuit of the automatic transmission. In order to be possible to unlock the activated parking brake in case of drop of the electric voltage supply, or defect of the electronic control unit of the automatic transmission, or defective supply of transmission oil, an emergency unlocking device is disclosed by means of which the parking brake can be mechanically unlocked.
DE 198 34 156.3 describes a parking brake system for an automatic transmission of a motor vehicle where the parking brake detent pawl is engaged as result of displacement by a spring accumulator and unlocked by hydraulic pressure made available via the oil pressure circuit of the automatic transmission under simultaneous prestress of the spring accumulator. To prevent an undesired engagement of the parking brake when the hydraulic pressure is cut off, there is provided a mechanical ball interlocking of the hydraulic actuating device of the parking brake. The ball interlocking is actuated, via an electromagnet, and kept in locking position so that the parking brake remains in unlocked position as long as the magnet is supplied with current.
The preamble of DE 41 27 991 C2 describes a parking brake system for an automatic transmission of a motor vehicle having an electrically constructed operative connection between the control device of the automatic transmission in the interior of the vehicle and the shiftable mechanism of the parking brake in which the parking brake is activated via a mechanical spring accumulator and deactivated via an electrohydraulic control device with control medium overpressure. For this purpose, the actuating rod with which the parking brake is brought to braking position and released from the braking position is designed at the same time as piston rod for an easily operating cylinder. To deactivate the parking brake and to keep the parking brake in the deactivated state, the cylinder is loaded with pressure oil via a solenoid valve. The hydraulic pressure for this is preferably supplied directly via the pressure oil circuit of the automatic transmission which also supplies the hydraulic transmission control with pressure oil. In the selector lever position P, the solenoid valve is in braking position and aerates the cylinder. In the pressureless state, the cylinder moves the parking brake to braking position by the spring of the spring accumulator. In one development is additionally proposed to provide an overpressure accumulator with shut-off valve in order to be able to hold the parking brake in deactivated state within a limited period of time while the engine is stationary and thus lacking pump pressure. Instead of the oil pressure supply, a pneumatic pressure supply can also be provided.
JP 58-110344 A has disclosed a parking brake system for a transmission where, in addition to a mechanical actuating device of a parking brake, there is provided an automatic, pneumatic actuating device of parking brakes. The parking brake is locked here via a spring accumulator acting upon a bolt which upon actuation of the parking brake engages in the toothing of a common gear of parking brakes. The additional pneumatic actuating device likewise acts upon the bolt and is controlled via a solenoid valve in accordance with driving conditions, the pneumatic energy being taken as negative pressure from a suction system of the prime mover. The additional pneumatic actuating device of the parking brake works, therefore, as automatic locking brake of the vehicle in the normal driving operation.
In the systems already described, it is disadvantageous that in case of failure of the pressure supply of the parking brake system, that is for example, when the engine stops or when the pressure circuit of the automatic transmission fails, the parking brake then actuated can no longer be released without a mechanical emergency unlocking device. If a mechanical emergency unlocking device of the parking brake is provided, which is accessible only from outside the interior of the vehicle, for example, in the engine space, and even if an acoustic coupling, such as by Bowen cable, from the transmission to the interior is prevented, the control thereof is rather circumstantial. Besides, the driver cannot immediately stop a vehicle inadvertently rolling back during actuation of the emergency unlocking device. A mechanical emergency unlocking device of the parking brake, accessible from the interior of the vehicle, in turn, has the known acoustic disadvantages of coupling of the bone connection of the transmission to the interior of the vehicle.
The problem on which the invention is based is to improve a parking brake system which has an electric operative connection between parking brake and control device thereof in the interior of the motor vehicle, as spring accumulator for activating the parking brake and an electrohydraulic or electropneumatic device for deactivating the parking brake, in the sense that a release of the parking brake when the prime mover of the motor vehicle is stationary and when the pressure supply of the transmission fails is possible without a mechanical emergency unlocking device of the parking brake.
This problem is solved according to the invention by a parking brake system having the features stated in the main claim. Other advantageous developments of the invention result from the sub-claims
When the prime mover of the motor vehicle is stationary, the pressure oil pump usually connected with the engine shaft is also stationary. The pressure oil system of the transmission thus can deliver no direct energy via the oil pump for an active actuation of the release device of the parking brake. An overpressure accumulator integrated in the pressure circuit of an automatic transmission such as proposed in DE 41 27 991 C2, due to inner leakages determined by the system, will not be able already after a relatively short time to deliver enough actuating pressure. Other overpressure sources connected with the prime mover such a pneumatic compressor for pressurizing with compressed air automated gear selector systems or a high-pressure servo-pump of a braking power system thus cannot be used as energy sources for the release device of the parking brake. A passive actuation of the release device of the parking brake, for example, via an added spring accumulator which acts upon pressure failure, is critical for reasons of safety and can lead to undesired rolling back of the vehicle.
The use of electric energy stored in accumulators such as usually available in the motor vehicle for actuating the release device of the parking brake would in an emergency imply relatively great current intensities especially when the parking brake is braced, for example, when deactivating the parking brake on the slope. Besides, additional parts are needed, for example, actuating magnet or electric motor with reduction transmission.